Multi-array laterolog tools and methods

ABSTRACT

Multi-array laterolog tool systems and methods acquire a set of array measurements sufficient to provide laterolog tool measurements of differing array sizes. Such systems and method offer multiple depths of investigation while offering greater measurement stability in borehole environments having high resistivity contrasts. In at least some system embodiments, a wireline or LWD tool body has a center electrode positioned between multiple pairs of guard electrodes and a pair of return electrodes. The tool&#39;s electronics provide a current from the center electrode to the pair of return electrodes and currents from each pair of guard electrodes to the pair of return electrodes. Each of the currents may be distinguishable by frequency or distinguishable by some other means. This novel arrangement of currents provides a complete set of measurements that enables one tool to simultaneously emulate a whole range of laterolog tools.

BACKGROUND

Modern oil field operators demand access to a great quantity of information regarding the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can he performed by several methods including wireline logging and “logging while drilling” (LWD).

In wireline logging, a sonde is lowered into the borehole after some or all of the well has been drilled. The sonde hangs at the end of a long wireline cable that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.

In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated, thereby enabling measurements of the formation while it is less affected by fluid invasion. While LWD measurements are desirable, drilling operations create an environment that is generally hostile to electronic instrumentation, telemetry, and sensor operations.

Among the available wireline and LWD tools are a variety of resistivity logging tools including, in particular, “array laterolog” tools. Such tools typically include a central electrode around a tool body, with guard electrodes symmetrically spaced above and below the central electrode. The tool drives auxiliary currents between the guard electrodes and the center electrode to “focus” the current from the center electrode, i.e., to reduce dispersion of the current from the center electrode until after the current has penetrated some distance into the formation. Generally speaking, a greater depth of investigation can be achieved using more widely-spaced guard electrodes, but the vertical resolution of the measurements may suffer. Accordingly, existing tools employ multiple sets of guard electrodes at different spacings from the central electrode to enable multiple depths of investigation without unduly sacrificing vertical resolution. Laterolog tools with one, two, three, and four sets of guard electrodes have been created. Though measurements of the simpler tools are conceptually subsets of the measurements provided by the more complex tools, in practice the presence of the extra guard electrodes affects the measurements of the complex tools, thereby making it difficult to compare measurements from different tools.

DESCRIPTION OF THE DRAWINGS

The various disclosed embodiments are better understood when the following detailed description is considered in conjunction with the accompanying drawings, in which:

FIG. 1 shows an illustrative environment for logging while drilling (“LWD”);

FIG. 2 shows an illustrative environment for wireline logging;

FIG. 3 shows an illustrative environment for tubing-conveyed logging;

FIG. 4 is a block diagram of an illustrative multi-array laterolog tool;

FIG. 5 shows an illustrative multi-array laterolog tool;

FIG. 6 illustrates a current flow pattern for a laterolog tool with six sets of guard electrodes;

FIGS. 7A-7E illustrates the current flow patterns that can be derived from measurements of the FIG. 6 current flow pattern;

FIG. 8 illustrates one method for deriving the flow pattern of FIG. 7A;

FIG. 9 illustrates a derivation of the flow pattern of FIG. 7B; and

FIG. 10 is a flow diagram of a multi-array laterolog logging method.

While the invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and will herein he described in detail. It should be understood, however, that the drawings and detailed description are not intended to limit the disclosure, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the appended claims.

DETAILED DESCRIPTION

Accordingly, there are disclosed herein improved multi-array laterolog tool systems and methods that acquire a set of array measurements sufficient to provide laterolog tool measurements of differing array sizes. Such systems and method offer multiple depths of investigation while offering greater measurement stability in borehole environments having high resistivity contrasts. In at least some system embodiments, a wireline or LWD tool body has a center electrode positioned between multiple pairs of guard electrodes and a pair of return electrodes. The tool's electronics provide a current from the center electrode to the pair of return electrodes and currents from each pair of guard electrodes to the pair of return electrodes. Each of the currents may be distinguishable by frequency or distinguishable by some other means. This novel arrangement of currents provides a complete set of measurements that enables one tool to simultaneously emulate a whole range of laterolog tools. The contemplated numbers of guard electrode pairs ranges from three to five, though of course more can be employed if space permits. Monitor electrodes can also be provided to separate the signal measurements from the transmission source.

At least some method embodiments include operating on measurements indicative of electrical resistance between a pair of return electrodes and each of: a center electrode and multiple pairs of guard electrodes. The measurements are processed to determine formation resistivity logs at multiple depths of investigation. One processing approach includes deriving measurements of resistivity between the center electrode and each pair of guard electrodes before combining the derived measurements in a manner that enforces predetermined relationships between the voltages at the monitor electrodes. Examples of the predetermined relationships include: equality, and a predetermined ratio.

The disclosed systems and methods are best understood in the context of the larger systems in which they operate. Such systems are illustrated in FIGS. 1-3.

FIG. 1 shows an illustrative logging while drilling (LWD) environment. A drilling platform 2 is equipped with a derrick 4 that supports a hoist 6 for raising and lowering a drill string 8. The hoist 6 suspends a top drive 10 suitable for rotating the drill string 8 and lowering the drill string through the well head 12. Connected to the lower end of the drill string 8 is a drill bit 14. As bit 14 rotates, it creates a borehole 16 that passes through various formations 18. A pump 20 circulates drilling fluid through a supply pipe 22 to top drive 10, down through the interior of drill string 8, through orifices in drill bit 14, back to the surface via the annulus around drill string 8, and into a retention pit 24. The drilling fluid transports cuttings from the borehole into the pit 24 and aids in maintaining the integrity of the borehole 16. Various materials can be used for drilling fluid, including a salt-water based conductive mud.

A LWD tool suite 26 is integrated into the bottom-hole assembly near the bit 14. As the bit extends the borehole through the formations, logging tool 26 collects measurements relating to various formation properties as well as the tool orientation and various other drilling conditions. The LWD tools 26 may take the form of a drill collar, i.e., a thick-walled tubular that provides weight and rigidity to aid the drilling process. (For the present discussion, the set of logging tools is expected to include a multi-array laterolog resistivity tool to measure formation resistivity.) A telemetry sub 28 may be included to transfer images and measurement data to a surface receiver 30 and to receive commands from the surface. In some embodiments, the telemetry sub 28 does not communicate with the surface, but rather stores logging data for later retrieval at the surface when the logging assembly is recovered.

At various times during the drilling process, the drill string 8 may be removed from the borehole as shown in FIG. 2. Once the drill string has been removed, logging operations can be conducted using a wireline logging sonde 34, i.e., a probe suspended by a cable 42 having conductors for transporting power to the sonde and telemetry from the sonde to the surface. A wireline logging sonde 34 may have pads and/or centralizing springs to maintain the tool near the axis of the borehole as the tool is pulled uphole. Logging sonde 34 can include a variety of sensors including a multi-array laterolog tool for measuring formation resistivity. A logging facility 44 collects measurements from the logging sonde 34, and includes a computer system 45 for processing and storing the measurements gathered by the sensors.

An alternative logging technique is tubing-conveyed logging. FIG. 3 shows an illustrative coil tubing logging system in Which coil tubing 54 is pulled from a spool 52 by a tubing injector 56 and injected into a well through a packer 58 and a blowout preventer 60 into the well 62. In the well, a supervisory sub 64 and one or more logging tools 65 are coupled to the coil tubing 54 and configured to communicate to a surface computer system 66 via information conduits or other telemetry channels. An uphole interface 67 may be provided to exchange communications with the supervisory sub and receive data to be conveyed to the surface computer system 66.

Surface computer system 66 is configured to communicate with supervisory sub 64 to set logging parameters and collect logging information from the one or more logging tools 65 such as a multi-array laterolog tool. Surface computer system 66 is configured by software (shown in FIG. 3 in the form of removable storage media 72) to monitor and control downhole instruments 64, 65. System 66 includes a display device 68 and a user-input device 70 to enable a human operator to interact with the system control software 72.

In each of the foregoing logging environments, the logging tool assemblies may include a navigational sensor package having directional sensors for determining the inclination angle, the horizontal angle, and the rotational angle (a.k.a. “tool face angle”) of the bottomhole assembly (BHA). As is commonly defined in the art, the inclination angle is the deviation from vertically downward, the horizontal angle is the angle in a horizontal plane from true North, and the tool face angle is the orientation (rotational about the tool axis) angle from the high side of the wellbore. In accordance with known techniques, directional measurements can be made as follows: a three axis accelerometer measures the earth's gravitational field vector relative to the tool axis and a point on the circumference of the tool called the “tool face scribe line”. (The tool face scribe line is typically drawn on the tool surface as a line parallel to the tool axis.) From this measurement, the inclination and tool face angle of the BHA can be determined. Additionally, a three axis magnetometer measures the earth's magnetic field vector in a similar manner. From the combined magnetometer and accelerometer data, the horizontal angle of the BHA may be determined.

A discussion of the multi-array laterolog tool electronics is in order before describing the physical construction of the tool. FIG. 4 shows a functional block diagram of the tool electronics. The control module 410 governs the operation of the tool in accordance with software and/or firmware 412 stored in internal memory. The control module 410 couples to telemetry module 420 to receive commands and to provide measurement data. Control module 410 further connects to digital-to-analog converter 430 to drive current electrodes 432, and connects to analog-to-digital converter 440 to make voltage measurements via monitor electrodes 442. Control module 410 can be, for example, a general purpose processor, a digital signal processor, a programmable gate array, or an application specific integrated circuit. Telemetry module 420 receives and stores measurement data in a nonvolatile memory 422, and further operates as a communications interface between the control module 410 and the telemetry communications mechanism.

FIG. 4 shows 2N+1 current electrodes (electrodes A0, A1, A2, . . . AN, A1′, A2′, . . . AN′) being independently driven via digital-to-analog converter 430. In some tool embodiments, the guard electrodes are electrically connected in pairs, i.e., electrode A1 is connected to electrode A1′ by an electrical conductor, electrode A2 is connected to electrode A2′, etc., Moreover, the return electrodes are electrically connected (i.e., electrode AN is conductively coupled to electrode AN′). In such alternative embodiments, the digital-to-analog converter can be simplified to drive only one electrode in each pair. Similarly, the monitor electrodes 442 can be electrically connected in pairs, i.e., with electrode M1 connected to M1′, electrode M2 connected to M2′, etc. It is also contemplated that each electrode can be individually driven/sensed and that the control module can collect the pair-wise measurements by appropriately combining the individual electrode currents and voltages.

FIG. 5 shows an illustrative multi-array laterolog tool 502 having (for the sake of illustration) equally-spaced current electrodes and return electrodes (electrodes A0, A1, A2, . . . A6, A1′, A2′, . . . A6′), with interspersed monitor electrodes M1-M10 and M1′-M10′ on a wireline tool body. (It is desirable to separate the monitor electrodes from the current electrodes because the current electrodes often develop an impedance layer that distorts voltage measurements when current is flowing) In practice, the electrodes are not equally sized and spaced. Better performance is achieved by having the more distant electrode increase in size. Thus, in one contemplated embodiment the center electrode A0 has an axial length of 6 inches. The lengths of electrodes Ai and Ai′ for i ranging from 1 to 6 is (in inches) 6, 8, 10, 14, 20, and 75. The spacing between the current electrodes also increases, beginning at 6 inches between electrodes A0 and A1, 6 inches between electrodes A1 and A2, 10 inches between electrodes A2 and A3, 14 inches between A3 and A4, 19 inches between A4 and A5, and 34 inches between A5 and A6. (These spacings are measured between the nearest edges and not center to center, and they are symmetric with respect to the center electrode.) In this contemplated embodiment, each of the monitor electrodes has an axial length of 1 inch. With one exception, the monitor electrodes are spaced 1 inch away from the nearest current electrode. (Electrodes M2 and M2′ are spaced 2 inches from current electrodes A1 and A1′, respectively.)

The tool electronics employ the current electrodes to provide the currents I0−I5 and I0′−I5′ as illustrated in FIG. 6. Currents I0−I5 are sourced from electrodes A0-A5 respectively, with electrode A6 serving as a common return electrode for each of these currents. Similarly, currents I0′−I5′ are sourced from electrodes A0 and A1′-A5 respectively, with electrode A6′ serving as a common return electrode for these currents. If the current and monitor electrodes are pair-wise connected as discussed before, the tool cannot distinguish currents I0−I5 from I0′−I5′, but instead operates on the combined currents (I0+I0′, I1+I1′, I2+I2′, . . . ). Otherwise, the tool can analyze currents I0−I5 separately from I0′−I5′, or in the alternative, combine the currents and voltages digitally before analyzing. Due to the symmetry of the array, only the top half is illustrated in the ensuing figures. Though not shown, the bottom half is presumed to be present.

To enable the monitor electrodes to distinguish the effects of the various currents, the currents are given distinguishable features. In the contemplated tool embodiment, the electrodes are pair-wise connected and currents I0−I5 have distinguishable signal frequencies f1-f6. The contemplated set of frequencies includes 80 Hz, 115 Hz, 145 Hz, 185 Hz, 235 Hz, and 285 Hz. (It is expected that the borehole fluid will he fairly conductive, thereby allowing low frequency currents to pass into and through the formation.) This frequency set offers sufficient frequency spacing to enable fast logging, while not spreading the frequencies so far apart as to incur excessive frequency dependence in the resistivity measurements. Moreover this frequency set avoids the use of harmonic frequencies which could he unduly sensitive to nonlinear effects in the system. Nevertheless, other sets of frequencies would also be suitable for distinguishing the currents. Alternatively, the currents could be distinguished through the use of time division multiplexing, code division multiplexing, or other methods that enable the currents to he independently monitored.

While each of the currents is provided with a characteristic that makes its effects distinguishable from those of the other currents, in at least some tool embodiments some of the currents are given common features. For example, some tool embodiments provide current I0 with frequencies f1 and f2. The sharing of frequency f2 by both current I0 and I1 enables straightforward hardware focusing as described in greater detail below.

As the tool drives the current electrodes, the currents pass through the borehole fluid and the formation to reach the return electrodes, creating a field potential indicative of the resistivity of the materials along the various current flow paths. The control module records a voltage signal from each monitor electrode to measure the field potential at the monitor electrode locations. A frequency analysis of the voltage signals (e.g., by Fourier transform, filtering, or least-squares curve fitting) separates out those voltage signal components attributable to each of the currents.

With the measurements for the current flow pattern of FIG. 6, it becomes possible to derive the measurements associated with each of the current flow patterns provided in FIGS. 7A-7E. FIG. 7E represents the full array measurement (which corresponds to the actual current flow pattern of FIG. 6), while FIGS. 7A-7D represent truncated array measurements of various degrees. In FIG. 7A (sometimes referred to below as Model 1), current electrode A2 is the shared return electrode, whereas in FIG. 7B (Model 2), current electrode A3 is the shared return electrode, and so on. By determining measurements for each of the array sizes, the tool can provide resistivity measurements not only as a function of tool position, but also as a function of radial distance from the borehole.

FIG. 8 demonstrates one approach for deriving the tool measurements of FIG. 7A (Model 1). From the measurements for the complete set of currents 802, the measurements corresponding to currents I0, I1, and I2 are extracted (label 804), e.g., by identifying those components of the monitor electrode voltage signals having the corresponding frequency components. The difference between the measurements for the desired truncated current flow pattern 808 and the extracted measurements 804 is that set of voltage measurements that would be obtained in response to the current flow pattern 806, which can be readily derived from the measurements corresponding to current I2. Representing the extracted measurements for monitor electrode Mi in vector form: v _(i) =[v _(i,1) v _(i,2) v _(i,3)]  (1) where the different vector components correspond to different frequencies f1, f2, f3. (Throughout the following description, v_(i,j) represents the jth frequency component of the voltage signal received by the ith monitor electrode.) If the currents I0, I1, and I2 differ only in frequency and not magnitude, then the truncated flow pattern measurements 808 are: v′ _(i) =[v _(i,1) −v _(i,3) v _(i,2) −v _(i,3) v _(i,3) −v _(i,3)]  (2) The last vector component is of course zero, as I2 is not part of the truncated flow pattern. (Where the current magnitudes are not equal the measurements should be scaled accordingly before applying the correction.)

Those familiar with laterolog tools recognize that the analysis is not yet complete, as the tool has not yet provided for focusing of the current. As with existing laterolog tools, focusing is accomplished by balancing the current from the center electrode with currents from the guard electrodes. In the current flow pattern of FIG. 7A, the proper balance had been achieved when monitor electrodes M1 and M2 have signals with equal contributions from each current, i.e., when: v′ _(1,1) −v′ _(2,1) =c(v′ _(2,2) −v′ _(1,2))  (3) where c is a scale factor that causes I1 to balance I0. (This use of internal scale factors is herein termed “software focusing”) The apparent resistivity is then

$\begin{matrix} {R_{{Model}\mspace{11mu} 1} = {k_{1}\frac{V_{M\; 1}}{I_{0}}}} & (4) \end{matrix}$ where k1 is a tool constant, I0 is the current from the center electrode, and V _(M1) =v′ _(1,1) +cv′ _(1,2).  (5)

The foregoing approach can be condensed into the following equations

$\begin{matrix} {{{\begin{bmatrix} {v_{1,2} - v_{2,2}} & {v_{1,3} - v_{2,3}} \\ I_{1,2} & I_{2,3} \end{bmatrix}\begin{bmatrix} c_{1,1} \\ c_{1,2} \end{bmatrix}} = \begin{bmatrix} {v_{2,1} - v_{1,1}} \\ {- I_{0,1}} \end{bmatrix}},{and}} & (6) \\ {{v_{M\; 1} = {v_{1,1} + {c_{1,1}v_{1,2}} + {c_{1,2}v_{1,3}}}},} & (7) \end{matrix}$ in combination with equation (4) above. The first and second subscripts in I_(j,k) are the source electrode (A0, A1, . . . ) and the frequency (f1, f2, . . . ). The second subscript is added for generality. Certain tool embodiments employ currents with multiple frequency components to enable adaptive hardware balancing of the currents. For example, the current from electrode A0 can include two signal frequencies f1 and f2, where f2 is also the signal frequency of the current from electrode A1. The control module 410 (FIG. 4) can then dynamically adjust the relative magnitude of currents I0 and I1 to ensure that v_(1,2)=v_(2,2), a technique which is herein termed “hardware focusing”. Hardware focusing is expected to improve measurement stability. Nevertheless, to obtain the measurements of a truncated array laterolog, software focusing is still used. In this example the equations become:

$\begin{matrix} {{{\begin{bmatrix} {v_{1,1} - v_{2,1}} & {v_{1,3} - v_{2,3}} \\ I_{0,1} & I_{2,3} \end{bmatrix}\begin{bmatrix} c_{1,1} \\ c_{1,2} \end{bmatrix}} = \begin{bmatrix} {v_{2,2} - v_{1,2}} \\ {- \left( {I_{0,2} + I_{1,2}} \right)} \end{bmatrix}},} & (8) \end{matrix}$ with V _(M1) =c _(1,1) v _(1,1) +v _(1,2) +c _(1,3) v _(1,3).  (9) and I ₀ =c _(1,1) I _(0,1) +I _(0,2)  (10) in combination with equation (4) above.

Thus suitable equations for the software focusing approach and the combined hardware/software focusing approach for Model 1 have been disclosed. FIG. 9 demonstrates a similar approach for deriving the tool measurements of FIG. 7B (Model 2). From the measurements for the complete set of currents 802, the measurements corresponding to currents I0, I1, I2, and I3 are extracted (label 904), e.g., by identifying those components of the monitor electrode voltage signals having the corresponding frequency components. With the correction representing the current flow pattern 906, the truncated current flow measurements 908 can be found. The equations for the software-focused approach are:

$\begin{matrix} {R_{{Model}\mspace{11mu} 2} = {k_{2}\frac{V_{M\; 1}}{I_{0}}}} & (11) \end{matrix}$ where k2 is the tool constant for Model 2 and V _(M1) =v _(1,1) +c _(2,1) v _(1,2) +c _(2,2) v _(1,3) +c _(2,3) v _(1,4),  (12) with the coefficients being the solution to the simultaneous set of equations

$\begin{matrix} {\begin{bmatrix} {v_{1,2} - v_{2,2}} & {v_{1,3} - v_{2,3}} & {v_{1,4} - v_{2,4}} \\ {v_{3,2} - {\lambda_{2,2}v_{4,2}}} & {v_{3,3} - {\lambda_{2,2}v_{4,3}}} & {v_{3,4} - {\lambda_{2,2}v_{4,4}}} \\ I_{1,2} & I_{2,3} & I_{3,4} \end{bmatrix}{\quad{\begin{bmatrix} c_{2,1} \\ c_{2,2} \\ c_{2,3} \end{bmatrix} = \mspace{475mu}{\quad{\left\lbrack \begin{matrix} {v_{2,1} - v_{1,1}} \\ {{\lambda_{2,2}v_{4,1}} - v_{3,1}} \\ {- I_{0,1}} \end{matrix} \right\rbrack.}}}}} & (13) \end{matrix}$ This last set of equations introduces the use of an enhanced focusing factor λ_(m,n), which is a desired ratio between the voltages on selected monitor electrodes. The first subscript m is the model number (2 indicates Model 2) and the second subscript n is the position of the monitor electrodes (n=2 indicates the monitor electrodes between current electrodes A1 and A2, whereas n=3 indicates the monitor electrodes between A2 and A3, n=4 indicates the monitor electrodes between A3 and A4, and n=5 indicates the monitor electrodes between A4 and A5). The focusing factor is a selected ratio between the inner monitor electrode and the outer monitor electrode (e.g., λ_(2,2) is the selected ratio of M3 to M4). By default, the selected value of λ is 1.0 (meaning enforced equality), but it has been found that often a deeper depth of investigation can be achieved by decreasing λ by around 10 to 20 percent.

For the combined hardware/software focusing approach (where the tool sources 10 with two frequency components as described previously), equation (10) is used in combination with the following equations: V _(M1) =c _(2,1) v _(1,1) +v _(1,2) +c _(2,2) v _(1,3) +c _(2,3) v _(1,4),  (13) I ₀ =c _(2,3) I _(0,3) +I _(0,2)  (14) with the coefficients being the solution to the simultaneous set of equations

$\begin{matrix} {\begin{bmatrix} {v_{1,1} - v_{2,1}} & {v_{1,3} - v_{2,3}} & {v_{1,4} - v_{2,4}} \\ {v_{3,1} - {\lambda_{2,2}v_{4,1}}} & {v_{3,3} - {\lambda_{2,2}v_{4,3}}} & {v_{3,4} - {\lambda_{2,2}v_{4,4}}} \\ I_{0,1} & I_{2,3} & I_{3,4} \end{bmatrix}{\quad{\begin{bmatrix} c_{2,1} \\ c_{2,2} \\ c_{2,3} \end{bmatrix} = {\begin{bmatrix} {v_{2,2} - v_{1,2}} \\ {{\lambda_{2,2}v_{4,2}} - v_{3,2}} \\ {- \left( {I_{0,2} + I_{1,2}} \right)} \end{bmatrix}.}}}} & (15) \end{matrix}$

For Model 3 (FIG. 7C) the equations for the software focused approach are:

$\begin{matrix} {R_{{Model}\mspace{11mu} 3} = {k_{3}\frac{V_{M\; 1}}{I_{0}}}} & (16) \end{matrix}$ where k3 is the tool constant for Model 3 and V _(M1) =v _(1,1) +c _(3,1) v _(1,2) +c _(3,2) v _(1,3) +c _(3,3) v _(1,4) +c _(3,4) v _(1,5),  (17) with the coefficients being the solution to the simultaneous set of equations

$\begin{matrix} {\begin{bmatrix} {v_{1,2} - v_{2,2}} & {v_{1,3} - v_{2,3}} & {v_{1,4} - v_{2,4}} & {v_{1,5} - v_{2,5}} \\ {v_{3,2} - {\lambda_{3,2}v_{4,2}}} & {v_{3,3} - {\lambda_{3,2}v_{4,3}}} & {v_{3,4} - {\lambda_{3,2}v_{4,4}}} & {v_{3,5} - {\lambda_{3,2}v_{4,5}}} \\ {v_{5,2} - {\lambda_{3,3}v_{6,2}}} & {v_{5,3} - {\lambda_{3,3}v_{6,3}}} & {v_{5,4} - {\lambda_{3,3}v_{6,4}}} & {v_{5,5} - {\lambda_{3,3}v_{6,5}}} \\ I_{1,2} & I_{2,3} & I_{3,4} & I_{4,5} \end{bmatrix}{\quad{\begin{bmatrix} c_{3,1} \\ c_{3,2} \\ c_{3,3} \\ c_{3,4} \end{bmatrix} = {\begin{bmatrix} {v_{2,1} - v_{1,1}} \\ {{\lambda_{3,2}v_{4,1}} - v_{3,1}} \\ {{\lambda_{3,3}v_{6,1}} - v_{5,1}} \\ {- I_{0,1}} \end{bmatrix}.}}}} & (18) \end{matrix}$

For the combined hardware/software focusing approach (where the tool sources I0 with two frequency components as described previously), equation (16) is used in combination with the following equations: V _(M1) =c _(3,1) v _(1,1) +v _(1,2) +c _(3,2) v _(1,3) +c _(3,3) v _(1,4) +c _(3,4) v _(1,5),  (19) I ₀ =c _(3,1) I _(0,3) +I _(0,2)  (20) with the coefficients being the solution the simultaneous set of equations

$\begin{matrix} {\begin{bmatrix} {v_{1,1} - v_{2,1}} & {v_{1,3} - v_{2,3}} & {v_{1,4} - v_{2,4}} & {v_{1,5} - v_{2,5}} \\ {v_{3,1} - {\lambda_{3,2}v_{4,1}}} & {v_{3,3} - {\lambda_{3,2}v_{4,3}}} & {v_{3,4} - {\lambda_{3,2}v_{4,4}}} & {v_{3,5} - {\lambda_{3,2}v_{4,5}}} \\ {v_{5,1} - {\lambda_{3,3}v_{6,1}}} & {v_{5,3} - {\lambda_{3,3}v_{6,3}}} & {v_{5,4} - {\lambda_{3,3}v_{6,4}}} & {v_{5,5} - {\lambda_{3,3}v_{6,5}}} \\ I_{0,1} & I_{2,3} & I_{3,4} & I_{4,5} \end{bmatrix}{\quad{\begin{bmatrix} c_{3,1} \\ c_{3,2} \\ c_{3,3} \\ c_{3,4} \end{bmatrix} = {\begin{bmatrix} {v_{2,2} - v_{1,2}} \\ {{\lambda_{3,2}v_{4,2}} - v_{3,2}} \\ {{\lambda_{3,3}v_{6,2}} - v_{5,2}} \\ {- \left( {I_{0,2} + I_{1,2}} \right)} \end{bmatrix}.}}}} & (21) \end{matrix}$

The equations for Models 4 and 5 can be readily written by observing the patterns made apparent in the equations for Models 1-3. Each column in the simultaneous-equation matrices corresponds to a signal frequency. The last row corresponds to the current components, while each of the remaining rows is the desired balance for the two monitor electrodes between a given set of current electrodes.

FIG. 10 provides an overview of a multi-array laterolog resistivity logging method. Beginning in block 1402, the tool is conveyed through a borehole containing a conductive fluid, The tool can be drawn through the borehole by a wireline cable, or conveyed on a tubing string, or incorporated into the bottom hole assembly of a drill string. In block 1404 the tool electronics energize the current electrodes to generate currents in the manner outlined previously. In block 1406, the tool or some other component of the system tracks the motion and/or position of the tool as the tool electronics sample the voltage signals from the monitor electrodes, in block 1408, the tool electronics record the voltage signals into an information storage medium and/or communicate the voltage signal data to a processing facility on the surface. In block 1410, the voltage signals are processed (downhole or at the surface) in accordance with one of the methods discussed above to determine the monitor electrode measurements and/or the generated currents (e.g., V_(M1) and/or I0) expected for each of the tool models 1-5 (see FIGS. 7A-7E). In block 1412, the voltage and current values for the various models are used to determine formation resistivity measurements at different depths of investigation (i.e., different effective radial measurement depths), enabling the logging system to determine a formation resistivity log that depends both on position along the borehole axis and on radial distance from the borehole axis. Some or all of the formation resistivity log data is displayed to a user in block 1414.

Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the claims be interpreted to embrace all such variations and modifications. 

What is claimed is:
 1. A resistivity logging system that comprises: a tool body having a center electrode positioned between multiple pairs of guard electrodes, wherein said multiple pairs of guard electrodes are positioned between a pair of return electrodes on said tool body; electronics that concurrently provide a primary current from the center electrode to the pair of return electrodes and a respective guard current from each pair in said multiple pairs of guard electrodes to the pair of return electrodes; and a processor that receives measurements indicative of electrical resistances associated with the concurrently provided primary current and respective guard currents, wherein the processor processes the measurements to determine measurements corresponding to a model having a reduced number of guard electrodes and a different return electrode position compared to said tool body.
 2. The system of claim 1, wherein said multiple pairs of guard electrodes includes at least three pairs.
 3. The system of claim 1, wherein said multiple pairs of guard electrodes include at least five pairs.
 4. The system of claim 1, wherein the electronics drive all of said primary and guard currents concurrently.
 5. The system of claim 4, wherein the primary current and each of the respective guard currents has a spectral signature that distinguishes it from other currents.
 6. The system of claim 4, wherein each of the guard currents has a unique signal frequency.
 7. The system of claim 6, wherein the primary current has at least one signal frequency different than each of the guard current signal frequencies.
 8. The system of claim 7, wherein the primary current further includes a signal frequency that matches a signal frequency of an innermost pair of guard electrodes.
 9. The system of claim 1, wherein the tool body further comprises at least one monitor electrode proximate to the center electrode and to each of said guard electrodes.
 10. The system of claim 1, wherein the processor processes the measurements to determine measurements corresponding to the model based on determining the difference between measurements from all of the guard electrodes and measurements from the reduced number of guard electrodes and performing focusing calculations for the reduced number of guard electrodes.
 11. The system of claim 1, wherein the tool body is a drilling collar.
 12. The system of claim 1, wherein the tool body is a wireline sonde.
 13. A resistivity logging method that comprises: receiving from a logging tool measurements indicative of electrical resistance between each of: a center electrode and a pair of return electrodes; an innermost pair of guard electrodes and said pair of return electrodes; and an outermost pair of guard electrodes and said pair of return electrodes, processing said measurements to determine a localized formation resistivity corresponding to a truncated array measurement based on a difference between measurements from all of the guard electrodes and measurements from a reduced number of guard electrodes and based on focusing calculations for the reduced number of guard electrodes, wherein the truncated array measurement corresponds to a model having the reduced number of guard electrodes and a different return electrode position compared to the logging tool; and displaying said localized formation resistivity as a function of position.
 14. The method of claim 13, wherein said processing comprises: deriving measurements indicative of electrical resistances between the center electrode and each pair of guard electrodes; and combining said derived measurements in a manner that enforces an assumption of commensurate voltages on the center electrode and at least one pair of guard electrodes.
 15. The method of claim 13, wherein said receiving further includes receiving measurements of electrical resistance between additional pairs of guard electrodes and said pair of return electrodes.
 16. The method of claim 13, wherein said measurements are voltages of monitor electrodes proximate to said center and guard electrodes, said guard electrode pairs and center electrode being driven by respective current sources.
 17. The method of claim 16, wherein the respective current sources provide currents with different signal frequencies.
 18. A non-transitory information storage medium that, when placed in operable relation to a computer, enables the computer to effect a method comprising: receiving measurements from a resistivity tool having a center current electrode between three or more pairs of current electrodes with increasing axial spacing between adjacent current electrodes on each side of the center current electrode, said measurements associated with concurrent current flows to an outermost pair of current electrodes from the center current electrode and from other pairs of current electrodes in said series; processing said measurements to obtain a log of formation resistivity as a function of tool position, said log corresponding to a truncated array measurement for a model having a reduced number of guard electrodes and a different return electrode position compared to the resistivity tool; and making said log available for viewing by a user.
 19. The medium of claim 18, wherein the log of formation resistivity indicates variation as a function of investigation depth.
 20. The medium of claim 18, wherein said processing includes determining corresponding measurements for current flows from the center current electrode to each pair of current electrodes. 